None of the 3 protocols you mentioned have much security built in. For example, POP3 and IMAP transmit clear-text authentication by default. However, you can wrap both in SSL for a marginal increase in security . SMTP has many issues, just think of all the SPAM you typically receive. If I had to choose between IMAP and POP3, I would go with IMAP over SSL, simply because it syncs messages and folder structures.